Apple guides development for WebKit and releases code for mDNSResponder (Avahi is admittedly used more in OSS operating systems, although NetBSD-current notably uses mDNSResponder), Darwin Streaming Server (the backend to their proprietary QuickTime Streaming Server, and ALAC.
They don't release everything as open source, but they certainly don't disregard its importance.
mDNSResponder is Apache. This was released to drive industry acceptance of Bonjour/Zeroconf.
Darwin Streaming Server is APSL (the Darwin license) and patent encumbered. IIRC, there was talk of it being open sourced to drive H264 acceptance.
Darwin mostly does count as sharing code for the sake of being open, and to their credit Apple does keep updating it, but I struggle to think of other Apple releases for which this appears to be the most important consideration.
Same goes for a company taking BSD/MIT licenced open source code, enhancing it and not releasing those enhancements back. In order to have those enhancements reinventing/duplication of effort is needed here aswell.
Also by dual-licencing your open source code you can provide it under GPL for FOSS projects while also offering a proprietary licence for companies for a fee. x264 makes good money this way.
This seems to be the most common GPL-based business model; particularly popular with Oracle, whose newest open-source/commercial license product (confusingly called Oracle NoSQL; it's more or less an autosharding network available BDB JE) is actually AGPL.